POWER December 2015 - 63

FUNDAMENTALS
3. Getting things back in line. This graph shows a typical relationship between fuel line distribution
and coal fineness. Finer coal generally distributes more evenly. Source: Storm Technologies
40
35
30
25
20
15
10
5
62
64
66
68
70
Pulverizer Mechanical Tuning Improvements
Worth Considering
There are various adjustments and mechanical
tuning measures that can be completed to improve
the performance of a modern coal pulverizer.
Locations identified on Figure 4 are keyed
to the following improvements (journal pressures
listed are for a #943-size pulverizer):
■ Install synchronized straight vane coarse
particle guide blades (A). The retrofit
lengthens the classifier blades, improves
material to 3/8-inch-thick AR400 or better,
and implements critical synchronization
of the classifier blades for fuel/air
two-phase mixture homogenization.
■ Increase journal preload (B) to between
36,000 lb and 42,000 lb (size appropriate
loading).
■ Install rotating throats (C) to ensure proper
velocity, improve primary classification,
and reduce coal rejects.
■ Ensure optimum bullring and roll geometry
(D) to achieve 1/4-inch roll-ring tolerances.
■ Install orifice housings (E) to support future
balancing efforts. The change offers
two advantages: It is easier for maintenance
personnel to change out orifice plates, and
it speeds testing and balancing efforts.
■ Install flared outlet cylinder extensions (F).
■ Repair all ceramic tile on the classifier cone
(G) to ensure a smooth inner diameter.
■ Install three new, or three exactly matched,
rolls (H) with similar 40,000 lb preload.
■ Verify that roll-to-ring clearances (I) are
absolutely no greater than 1/4-inch over
the full grinding length of the rolls and
that the clearance is parallel to the bowl
for the full width of the rolls.
■ Restore bowl angle (J) to the original design
angle, typically 20 degrees plus or
minus 1/2 degree.
■ Install designed throat deflectors (K) to
improve primary classifications and to
minimize issues with sand beds by vectoring
airflow towards the bullring.
■ Relocate primary air thermocouple to venturi
throat section (L), optimize insertion
December 2015 | POWER
72
74
76
78
Weighted average pulverizer coal fineness (% passing 200-mesh)
depth, and calibrate instrumentation more
accurately.
Additionally, ensure that venturi sensing
lines, connections, and transmitters are
all in good condition. Tempering air dampers
should be stroked and corrections made
to ensure that they close at least 99%. This
should also be done for hot air dampers.
All internal mill surfaces must be smooth so
that the swirl of the coal/air mixture may enter
and leave the classifier without spoiling or
turbulence caused by double layer tiles, welded
pad eyes, or other internal surface discontinuities.
This, combined with precise primary airflow
measurement and control, is important for
uniform fuel distribution at the classifier outlet.
All internal dimensions should be verified and
technically directed by a qualified service engineer
during installation of performance parts
and before closing the mill.
Other Opportunities for
Improvement
Overhauling Stock gravimetric feeders can
also be worthwhile. The refurbishment
should include calibrating load cells properly,
installing modern microprocessors, adjusting
belt tension appropriately, and completing
accurate speed calibration.
Another pulverizer performance monitoring
technique is to observe the drive motor power
input in correlation with the coal feed rate. The
relationship of ton/hour to kWh power input is
a very helpful leading indicator (Figure 5). A
reduction in the power used by a coal pulverizer
does not usually result in an improved heat rate.
Instead, more grinding power nearly always
correlates with better coal fineness. The only
exception is with a ball tube mill.
Getting Primary Airflow Right
Pulverizer capacity is not simply a measure
of coal throughput; capacity refers to a certain
coal throughput at a given fineness, raw
coal sizing, HGI (Hardgrove Grindability Index),
and moisture. Often, if the desired coal
throughput or load response is not achieved,
the primary airflow will be elevated to higher
flow rates than are best for capacity. However,
increased throughput achieved in this way
sacrifices fuel fineness (Figure 6).
This is very common. When the primary
airflow is higher than optimum, it creates entrainment
of larger-than-desired coal particles
leaving the mills, promotes poor fuel distribution,
lengthens flames, and impairs low-NOx
burner performance. We have found that targeting
an A/F ratio around 1.8 lb of air per lb of
■ HP/ton ■ Fineness
5. Mill power to ton of coal throughput. In general, when the mill draws more power
per ton of fuel, it is an indication that finer coal is being produced. Source: Storm Technologies
11.5
11.0
10.5
10.0
9.5
9.0
8.5
8.0
7.5
7.0
Mill A
Mill B
www.powermag.com
Mill C
Mill D
Mill E
Mill F
63
82
80
78
76
74
72
70
68
66
80
82
4. Fine tuning. Getting tolerances back
in specification by mechanically adjusting or
replacing some components can greatly improve
coal pulverizer performance. Source:
Storm Technologies
Maximum fuel imbalance
(% deviation from the mean)
HP/ton
% passing 200-mesh

http://www.powermag.com

POWER December 2015

Table of Contents for the Digital Edition of POWER December 2015